The backup could not be started because of an unexpected error in virtual disk service (0x80070057) (0x086C6)

This article describes the symptoms, cause and resolution for an issue that occurs on MARS Agent installations within Azure Virtual Machines.

Symptoms

Scheduled or adhoc backups on the MARS Agent installed in Azure Virtual Machines fail with the following error:
The backup could not be started because of an unexpected error in the virtual disk service. Restart the virtual disk service and try the backup operation again. If the issue persists, check the system event log for virtual disk service events. (0x086C6)

Here is the screenshot of the error on the MARS Agent job details dialog box:

Here is a screenshot of the error detail blade for the failed backup job on the Azure Portal:

Cause

The reason for the error is due to transient issues that are observed in the Windows Server/Client storage layers that prevent the backup metadata VHD from being mounted properly.

While we, at Microsoft are actively looking to fix this issue, please refer to the resolution below to mitigate this problem and ensure successful backups.

Resolution

The resolution is to move the MARS Agent scratch or cache location to a non-OS disk.During and after this move your backups, and their policy and schedule will be retained.